In optoelectronic distance measuring devices (EDM), an optical signal is emitted from the instrument in the direction of the target object—whose distance it is necessary to determine—for example as optical radiation in the form of laser light. In order that the point targeted for measurement on the target object is made discernible, visible light is often used in this case. The surface of the target object reflects at least part of the optical signal, usually in the form of a diffuse reflection. The reflected optical radiation is converted into an electrical signal by a photosensitive element in the instrument. With knowledge of the propagation velocity of the optical signal and with the aid of the ascertained propagation time required for covering the distance from the instrument to the target object and back, it is possible to determine the distance between instrument and target object. In this case, optical components for beam shaping, deflection, filtering, etc.—such as, for instance, lenses, wavelength filters, mirrors, etc.—are usually situated in the optical transmission and/or reception path.
In order to compensate for influences which might corrupt the measurement results (for example temperature influences, component tolerances, drifting of electronic components, etc.), part of the emitted optical signal can be guided as a reference signal via a reference path of known length from the light source to the light-sensitive receiving element. In this case, the reference path can be fixedly incorporated in the instrument or be designed as a deflection element that can be pivoted in or plugged on, for example. The reception signal resulting from said reference signal can be received by the photosensitive element used for measurement or by a dedicated photosensitive element. The resulting electrical reference signal can be used for referencing and/or calibrating the measured values ascertained.
In order to obtain a correspondingly high accuracy of the distance measurement, on account of the high propagation velocity of optical radiation, the requirements made of the temporal resolution capability for distance measurement are extremely high. By way of example, for a distance resolution of 1 mm, a time resolution having an accuracy of approximately 6.6 picoseconds is required.
With regard to the signal power that can be emitted, limits are predefined for the optoelectronic EDM under consideration here. In this regard, when laser light is emitted, eye safety determines a maximum permissible average signal power which is allowed to be emitted. In order nevertheless to obtain for the measurement sufficiently strong signal intensities which can be detected by the receiver, pulsed operation is therefore employed. The emitted optical signal is modulated in its intensity amplitude. Short pulses having a high peak power are emitted, followed by pauses without signal emission. Consequently, the reflected portion of the pulses has a sufficiently high intensity to be able to evaluate the latter from the background disturbances and noise, in particular even when background light (sunlight, artificial lighting, etc.) is present.
As described in EP 1 957 668, for instance, the emission of packets of pulses followed by pauses without pulse emission—so-called burst operation—especially affords not only the advantage of a reduced average power of the signal, but additionally also advantages in the achievable signal-to-noise ratio (SNR). Firstly, therefore, the signal intensity can be correspondingly higher during the active burst time than in the case of continuous emission—without exceeding the average power limit in the process. Secondly, moreover, the noise is taken up only in the time windows during the active burst duration—but not during the blanking intervals, since no signal evaluation takes place during the latter. By means of a duty cycle of the bursts e.g. of 0.1 or 1:10 or 10% (10% of the burst duration signal emission+90% pause), it is thus possible to achieve an improvement in the SNR of approximately the square root of 1/duty cycle, that is to say in the example of 10% an improvement by a factor of more than 3. The number of pulses per packet can be varied depending on the evaluation concept and measurement situation, through to individual pulses (in which case the term bursts is then generally no longer employed).
In order to ascertain the propagation time of the signal, firstly the so-called time-of-flight (TOF) method is known, which ascertains the time between the emission and reception of a light pulse, wherein the time measurement is effected with the aid of the edge, the peak value or some other characteristic of the pulse shape. In this case, pulse shape should be understood to mean a temporal light intensity profile of the reception signal, specifically of the received light pulse—detected by the photosensitive element. In this case, the point in time of transmission can be ascertained either with the aid of an electrical trigger pulse, with the aid of the signal applied to the transmitter, or with the aid of the reference signal mentioned above.
In the distance measurement, ambiguities can occur if the signal propagation time exceeds the reciprocal of the pulse or burst transmission rate and a plurality of signals are thus traveling simultaneously between instrument and measurement object, as a result of which a reception pulse or reception burst can no longer be assigned unambiguously to its respective transmission pulse or transmission burst. Without further measures it is thus unclear whether actually the determined distance or an integral multiple thereof was measured.
Secondly, the so-called phase measurement principle is known, which ascertains the signal propagation time by comparison of the phase angle of the amplitude modulation of the transmitted and received signals. In this case, however, the measurement result in the case of one transmission frequency has ambiguities in units of the signal period duration, thus necessitating further measures for resolving these ambiguities. By way of example, WO 2006/063740 discloses measurement with a plurality of signal frequencies which result in different unambiguity ranges, as a result of which incorrect solutions can be precluded. WO 2007/022927 is also concerned with unambiguities in phase measurement.
The described pulsed or burst operation of the distance measuring device requires the emission of short light pulses, that is to say a high modulation frequency of the emitted light. The EDM design presented below with specific numerical values shall be set out by way of example as one possible specific embodiment. The numerical values used in this case should be regarded in this case primarily as guide values for the orders of magnitude and size ratios of the signals with respect to one another, rather than as exhaustive, limiting indications of values. The orders of magnitude and numerical values indicated here are purely by way of example in order to illustrate a posed problem that is solved according to the invention. Practical examples of modulation frequencies in EDMs are in the 100 MHz to 10 GHz range, especially approximately 500 MHz to 1 GHz. In the case of transmitting elements in the form of semiconductors, this is obtained by means of correspondingly fast electrical driving. In order to achieve the required high peak power of the optical pulses in this case, therefore, the laser driver has to impress short electrical pulses having high current intensities into the light source.
By way of example, pulse durations in the range of nanoseconds or less, especially approximately 1 to 0.1 nanosecond or less, are customary values. The peak currents to be impressed in this case are in this case approximately in the range of some milliamperes, for example approximately 10 mA to 1 A, especially approximately 50 to 300 mA. In the case of an exemplary current pulse having a triangular shape and a peak value of 100 mA with a duration of 1 ns, this produces a current rise of an order of magnitude of 108 amperes per second. Given a total inductance of the drive circuit (in particular comprising laser diode, leads, driving circuit) of 10 nH, this already produces a voltage drop of the order of magnitude of the laser threshold voltage of the laser diode (of e.g. typically approximately 2.3V in the case of red laser diodes). In order for example to generate an approximately rectangular optical pulse, the requirements are correspondingly even higher, such that even a few nanohenries of lead inductance become apparent in the emitted pulse shape. Customary housing connections of laser diode components and also the bonding wires in the component themselves often already have an inductance of the order of magnitude of 5 to 10 nH, however, that is to say make up a considerable proportion of the inductive behavior of the drive circuit.
Purely in the context of the use of a laser driver or laser diode driver such as is disclosed for example in European patent application No. 11180282 (the content of which is incorporated by reference herein), a low lead capacitance of the laser diode is of importance. By virtue of the drive principle used therein with a supply voltage below the laser threshold voltage, parasitic inductances of the laser diode, or to put it more precisely of the leads thereof, have an even more problematic effect than in conventional laser drivers. However, in conventional laser diode drivers, too, a low inductance of the laser diode and the electrical connections thereof has a positive effect. Besides a metrological appreciation of this problem area, the topic of the lead inductance during the pulsed operation of laser diodes is also discussed for example in the TechNote TN #36000-2 from ILX Lightwave (www.ilxlightwave.com), wherein the dimensions and power classes described therein do not apply in the technical field of distance measuring devices. Especially for handheld, compact, battery-operated EDMs, the teaching presented therein is not applicable, in particular since the power classes are totally different and no standard connection cables described therein are used anyway.
The laser diodes according to the prior art, for instance as described in U.S. Pat. No. 7,192,201, have, owing to the dictates of construction, a parasitic inductance that is extremely high for the case of use described here. Contrary to the present application, said document even describes the introduction of an inductance into the laser diode.
In the case of such laser diodes used in the prior art, the parasitic inductance is high owing to the dictates of design, especially as a result of the comparatively long leads, bonding wires, etc. For high-frequency modulated EDMs, the inductive properties of these components are noticeably disturbing and should not be disregarded in the context of optimizing the EDM system.
Crosstalk effects in electronics also constitute a further limiting factor with regard to achieving high distance accuracies in the prior art. In this case, firstly, the transmitter with the short pulses having high amplitude values as desired in a manner governed by the use is a potential broadband interference source. In this case, the connections act as “transmitting antennas” which emit this electromagnetic interference.
CN 10 20 04 253 describes a distance measuring device comprising a laser diode, in which, opposite the primary measurement radiation emission, a smaller, secondary part of the radiation is likewise coupled out to a reference photodiode in order to form a reference path. The monitor diode usually fitted at this location in the case of laser diodes in the prior art is therefore replaced by a reference signal receiver. To put it another way, therefore, in this case the reference radiation is not derived at the front from the primary measurement beam, but rather at the back from the secondary monitor beam.